“Truth’’ dramatically recounts the story behind the “60 Minutes’’ segment that led Dan Rather (well-played by Robert Redford) to resign from CBS, but the brightest star of this great ensemble piece is Cate Blanchett, who may well end up with another Oscar nomination for playing Rather’s producer Mary Mapes.

Their joint fall from grace stemmed from a piece on President George W. Bush’s service in the Texas Air National Guard during the Vietnam War. They reported that Dubya barely showed up during his service and skipped a mandatory physical but suffered no punishment — and was actually granted early release while avoiding service in the war — because of his political connections.

The report aired two months before the 2004 elections, but Mapes and her team (portrayed by the trio of Dennis Quaid, Topher Grace and Elisabeth Moss) were a bit sloppy in their rush to meet a deadline. They failed to authenticate a key piece of evidence against Bush: an incriminating memo that CBS attributed to Bush’s by-then-deceased commanding officer, which was turned over to Mapes by a source (Stacy Keach) who insisted on anonymity.

Rather than disputing the whole story, the Republicans and rival news organizations quickly discredited the memo — of which CBS only possessed a faxed copy, several generations from the original — establishing that it had been written with a word-processing program rather than on a 1968-era typewriter.

The film, based on a memoir by Mapes, depicts how the story developed and CBS’s failed attempts at damage control once it blew up. There was a shameful attempt by Team Rather to shift blame to the source for the memo, who agreed to go public and admitted he lied about where he got the memo to protect his source.

Writer James Vanderbilt (“Zodiac’’), who is also making his impressive debut as a director on “Truth,’’ suggests that CBS ordered an independent investigation conducted by a Republican-leaning law firm of what happened to help its parent company, Viacom, seek political favors from the Bush administration.

While Redford does a fine job playing Rather, the film ignores the anchor’s famous showboating, temper and hubris, which may have contributed to his downfall. He resigned from CBS after being forced from his anchor chair after 24 years, but has been steadily employed elsewhere. Mapes, once one of the most respected field producers in the history of TV news, hasn’t worked in her field in 11 years.

“Truth,’’ which premiered over the weekend at the Toronto International Film Festival, opens Oct. 16.